Recently, a belt fixing type as illustrated in FIG. 11, for instance, is proposed as the fixing device utilizing the image heating apparatus that is used for the image forming apparatus of the electrophotographic type. This belt fixing type includes a heating roller 50, an endless belt 30 passed over the heating roller 50 and a separation roller 60 disposed separately from the heating roller 50 so as to rotate, and a pressure roller 70 for making the endless belt 30 pressure-contact the heating roller 50. In addition, the belt fixing type includes a cooling portion 80 for cooling a sheet retained by the endless belt 30 by contacting a belt inner circumferential surface of the endless belt 30 between the heating roller 50 and the separation roller 60. The heating roller 50 includes a roller core material 51, a heating halogen lamp 53 disposed inside the roller core material 51, and a coating layer 54 formed on the outer surface of the roller core material 51. Further, the pressure roller 70 includes a roller core material 71 and a coating layer 74 formed on the outer surface of the roller core material 71.
The endless belt 30 includes a belt substrate having a thickness of approximately 30 to 200 microns, and an elastic releasing layer having a thickness of approximately 10 to 200 microns formed on the outer circumferential surface of the belt substrate. The belt substrate is formed by using heat-resistant resin such as polyimide or polyamide and a metal material such as nickel or aluminum. In addition, the elastic releasing layer is formed by using silicone rubber, fluororubber or the like.
An operation of the fixing device having this structure is described.
The endless belt 30 is rotated in the direction indicated by the arrow “A” illustrated in FIG. 11, and a recording sheet P bearing a toner image T is fed to a pressure contact portion N between the endless belt 30 and the pressure roller 70. Then, the recording sheet P is retained on the surface of the endless belt 30 and passes through the cooling portion 80 so as to be cooled. The recording sheet P is further conveyed to the separation roller 60 and is separated from the endless belt 30. Those operations make the toner image T be fixed on the recording sheet P.
The cooling portion 80 cools the recording sheet P on which the toner image T is fixed at the pressure contact portion N1 via the endless belt 30, and hence the toner does not adhere to the endless belt 30 and an occurrence of offset can be prevented. In addition, since the recording sheet P is cooled after the toner image T is fixed on the recording sheet P in the state where the recording sheet P is retained on a flat belt surface, occurrence of a curl of the recording sheet P can be reduced. This technology is described in Japanese Patent Application Laid-Open No. 2004-203527. Note that the curl occurring in the recording sheet P at that time is mainly a curl in which the image side becomes concave due to the fixed toner that is shrunk more than the recording sheet when the recording sheet is cooled (hereinafter referred to as a toner curl).
Note that as the cooling portion there is one utilizing a cooling roller of an air cooling type that is disposed on the inner circumferential surface of the endless belt in a detachable manner, one utilizing an air cooling box having a lot of air passing holes formed on the surface contacting with the inner circumferential surface of the endless belt, or the like. Those technologies are described in Japanese Patent Application Laid-Open No. H04-216580 and Japanese Patent Application Laid-Open No. H05-72926.
Recently, however, various recording sheets having varieties of types, sizes and the like have become available in accordance with a purpose of use and the like of a user for the image forming apparatus such as a printer or a copying machine utilizing the electrophotographic method. For this reason, varieties of factors can affect a curl occurring in a recording sheet, and the factors include thickness of the recording sheet, density thereof, quality of material thereof, whether or not a coating layer is present, strength thereof, a grain direction thereof, contained moisture amount and the like.
In such the situation, it is difficult to prevent occurrence of the curl toward the image side of the recording sheet only by cooling and fixing the toner image in the cooling portion while the recording sheet is retained on the flat surface of the endless belt like the conventional fixing device.
In addition, recent printers and copying machines are required to have a higher productivity. When a conveying speed of the fixing belt is increased compared with the conventional one for instance, a cooling effect by the cooling portion may be decreased so that the effect of preventing occurrence of a curl may also be decreased.
Further, it can be considered to provide a curl correcting mechanism made up of a pair of rollers or the like on the downstream of the fixing device so that a generated curl can be corrected. When the curl correcting mechanism is used, however, the following problem may occur. When a curl correction is performed after the image side of the recording sheet is once separated from the endless belt, the image side may become coarse depending on surface smoothness of the curl correcting mechanism at a curl correction portion even when the image side has the same degree of smoothness as the endless belt due to the smoothness of the endless belt. In particular, when the recording sheet is not cooled sufficiently because of the increased conveying speed as described above, this problem may occur outstandingly.
Note that it may be considered to resolve the above-mentioned problem by adopting a curl correcting mechanism utilizing a belt having the same structure as the endless belt of the fixing device. However, the endless belt of the fixing device is usually expensive because the endless belt includes a belt substrate made of heat-resistant resin and a metal material such as nickel or aluminum, and an elastic releasing layer made of silicone rubber, fluororubber or the like formed on the belt substrate. Therefore, since cost of the apparatus may increase when the belt having the same structure as the endless belt is disposed for the curl correcting mechanism, the curl correcting mechanism usually utilizes a belt made of rubber or the like so that a high surface smoothness cannot be obtained.